Historic vehicle specialist Mike Williams Restorations has warned against the dangers of storing classic cars with unleaded fuel in the tank.

“When it becomes stale, in some cases, there is the potential for unleaded petrol to turn into a non-firing, corrosive liquid which wreaks havoc on fuel systems when the car is taken out of storage and started again,” said company founder Mike Williams, who has over 30 years’ experience in restoring and preparing historic road and race cars.

“Many owners of classic vehicles drive their cars only infrequently, and, while they may perform some general ‘’lay-up’ tasks when they put the car away for a few months, draining the fuel tank is unlikely to be one of them. Yet, as we, and other specialists we have spoken to, are seeing, this can be bad news – and it seems to be a growing problem. Even some manufacturers of modern road cars and agricultural equipment have issued service bulletins on the matter.”

One Mike Williams Restorations client booked his Aston Martin DB6 Vantage in for rectification of what was thought to be an electrical fault, which made starting the vehicle impossible. The car had been kept in a ‘storage bubble’ in the owner’s garage for the past year.

There was a fault in the starter circuit, but after this was rectified the engine still would not fire on six cylinders. Instead, it hissed and popped through the trumpets of its three Weber carburettors. The owner had previously seen flames spit from the carburettor trumpets and was naturally concerned about this apparent fire risk.

The carburettors leaked fuel into the airbox and the brass floats were green and corroded, the fuel smelt acrid and was opaque and the internal fuelways showed signs of corrosion and micro-debris. Even after the carburettors were removed, cleaned, and fitted with new floats and service kits, they still spat and fuel continued to leak.  The fuel tank was drained, and found to have more acrid-smelling fuel, plus what Mike describes as ‘something akin to French Dressing’, in the bottom of the tank, which was then flushed.

Fuel continued to leak through the carburettors, even when the engine had stopped and the ignition turned off. The fuel pump was rebuilt and refitted as some of the internal O-ring sealing washers had deteriorated. The fuel tank sender unit was found to be corroded and seized, but careful freeing, cleaning and working brought it back to life, and the car finally started as it should.

Mike Williams and his team discussed this with their suppliers, who, for many years, have supplied parts for SU equipment and also rebuild carburettors and fuel pumps themselves. The supplier said that they now fit fuel pump replacement seals which are made of Viton, as this is resistant to a wider range of fuel products. However, they frequently now see carburettors which are in an internally worse state than they used to see years ago; something they attribute to ‘degrading unleaded fuel’. And it appears that the difference in the formulation of fuels and how quickly they degrade is an ongoing problem.

“We feel this situation should be highlighted, as, when trying to analyse why a car won’t run, or runs poorly, it’s a new issue for we ‘fettlers’ to wrestle with – and it’s even more difficult to explain to clients, who will often say thing like ‘Well, it was running OK last year when I laid the car up’” said Mike Williams.

“Some owners think that they have got round the problem by using lead replacement additive but, while this can help with valve seat recession and ignition-related maladies, it does not necessarily prevent corrosion caused by degrading fuel, and that is a major issue. Part of the problem seems to be the inclusion of Ethanol in petrol nowadays, which, when it degrades, is not always compatible with many of the materials used in the fuel systems of older vehicles, so the use of a corrosion inhibitor becomes necessary. Owners who wish to address these problems need to take careful note of the various products available and whether the one they are considering deals with all of the potential problems noted, prior to mixing it with the fuel in their older vehicles.”

Mike Williams Restorations is making recommendations to its own clients as to the best course of action, and urges other owners of classic cars to visit the website of the Federation of British Historic Vehicle Clubs (, as this contains a great deal of useful information on the subject, which could save them both trouble and money.

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